Temperature-Adaptive Ultralubricity of a WS/a-C Nanocomposite Coating: Performance from Room Temperature up to 500 °C.

This study reports on the ultralubricity of a high-temperature resilient nanocomposite WS/a-C tribocoating. The coefficient of friction of this coating remains at around 0.02 independently of a thermal treatment up to ∼500 °C, as confirmed by high-temperature tribotests. Moreover, the coating annealed at 450 °C keeps exhibiting a similar ultralubricity when cooled back down to room temperature and tested there, implying a tribological self-adaptation over a broad temperature range. High-resolution TEM observations of the tribofilms on the wear track unveil that WS nanoplatelets form dynamically via atomic rearrangement and extend via unfaulting geometrical defects (bound by partial climb dislocations). The (002) basal planes of the WS nanoplatelets, reoriented parallel to the tribo-sliding direction, contribute to a sustainable ultralubricity. The declining triboperformance beyond 500 °C is associated with sulfur loss rather than the transformation of WS into inferior WO via oxidation as suggested earlier. This self-adaptive WS/a-C tribocoating holds promise for a constant ultralubrication with excellent thermal performance.